Hi

I fitted my M8100 mech today and all is well; shifting is a nice light but positive feel, no noise. However, I’m a bit confused about the low (b) screw adjustment. I set it originally before chain went on, but later, after I set the end adjustment screw properly with the chain in place, the b screw isn’t actually doing anything, I.e it is not touching the limit plate but the guide wheel is nowhere near touching the sprockets anywhere across the cassette.

Does anyone know if this is normal for this mech, it’s like the end adjustment screw did all the hard work? I’m just a bit paranoid I have done something wrong despite it all seeming to work OK! N.B. Hardtail if that’s relevant.

P.s. Of the many online resources, I found the Shimano dealer manual the most straight forward set of instructions to follow – especially for chain length.

Thanks

I’ve got the same derailleur but I’m not sure I know exactly which screw you are referring to.

There are 3 screws that matter: H, L & B.

H: Stops the chain from going off the smallest cog and jamming into the frame.
L: Stops the chain from going off the tallest cog and into your wheel/spokes.
B: Determines the distance between the upper jockey wheel and your cassette. I think ideally you want about 5-6mm gap.

So as long as H & L are set and the upper jockey wheel moves when you pedal then you’re doing fine.

I’ve found the following guide the best for all things reaer derailluer related. The video is very good at showing you which screw does what and how to adjust everything to get a nice clean shift:

https://www.parktool.com/blog/repair-help/rear-derailleur-adjustment

Full proper setup is detailed here : https://si.shimano.com/pdfs/dm/DM-MARD001-02-ENG.pdf

Chain length is super important to get correct and it’s not the same as an 11ap setup.
There is also a marking on the rear of the cage for setting the B limit correctly. The screw should definitely be doing (oh 19/20) something………

Hi, thanks for the reply. It’s the ‘low’ one that’s confused me. I understand it’s function – it’s just as soon as I wind it in to make any more than the slightest contact – its too much and I can’t get the 51t. So I have left it just touching (so if anything upsets the mech it can’t go beyond that point) and the gear changes are perfect. I shall worry about it no more!

Thanks again.

I also found cable tension is very fussy to get everything working properly. In the d d I used very little tension when I pulled the cable and tightened at the mech.

To much tension and the gears jumped.